This chapter discusses the microbiology of microbial fuel cells with emphasis on fuel cells powered by electricigens. Electricity production with electricigens is significantly different from that of other types of microorganisms. The ability of electricigens to directly transfer electrons to the anode surface also alleviates the need for unstable, and potentially toxic, mediators. The fact that, as far as is known, there has been no evolutionary pressure on microorganisms to produce electricity suggests that electricigens may not be optimized for electricity production. Introducing genes to increase production of the outer surface cytochrome, OmcS, or pilin did not show increased power production. Geobacter species are capable of accepting electrons from electrodes poised at low potential for the reduction of various electron acceptors. Geobacter-catalyzed reduction of U(VI) to U(IV) at electrode surfaces can precipitate uranium contamination from groundwater, precipitating U(IV) on the electrode. Of the microorganisms known to contribute to electricity production in microbial fuel cells, only electricigens offer the possibility of highly efficient, self-sustaining conversion of waste organic matter and renewal biomass to electricity. However, the study of electricity production with electricigens is clearly in its infancy.

Model for electron transfer to electrodes. (A) Low-power fuel cell. Microbes directly attached to electrodes can transfer electrons via outer surface cytochromes, such as OmcS, consistent with the omcS deletion mutant being greatly impaired in power production, but deletion of pilA having no impact. (B) High-power density fuel cell. Microbes directly attached to the anode can transfer electrons via outer surface cytochromes. Microbes not attached to the anode can transfer electrons to the electrode and/or other cells closer to the electrodes via conductive pili.

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Figure 2.

Model for electron transfer to electrodes. (A) Low-power fuel cell. Microbes directly attached to electrodes can transfer electrons via outer surface cytochromes, such as OmcS, consistent with the omcS deletion mutant being greatly impaired in power production, but deletion of pilA having no impact. (B) High-power density fuel cell. Microbes directly attached to the anode can transfer electrons via outer surface cytochromes. Microbes not attached to the anode can transfer electrons to the electrode and/or other cells closer to the electrodes via conductive pili.